Non-volatile dissolved organic iodine (DOI) can be a major, or even the dominant, species of dissolved I in coastal, inshore and estuarine waters. It can be converted to IO₃⁻ in the presence of an oxidizing agent and to I⁻ by reacting it with a reducing agent. Depending on the exact experimental conditions, the yields of these reactions may not be quantitative. In previous analytical schemes for the determination of IO₃⁻, I⁻ and DOI in marine waters, if oxidation or reduction steps are involved and the concentrations of one or more species are estimated by difference, the presence of DOI can lead to an overestimation of the concentrations of the inorganic species determined by difference and an underestimation of the concentration of DOI. In two cruises covering the James River to the southern Chesapeake Bay and from the southern Chesapeake Bay to the Atlantic, above a salinity (S) of 2, the contribution of DOI to total I increased with decreasing salinity and reached a maximum of 80%. DOI, I⁻ and IO₃⁻ were successively the dominant form of dissolved I at 0.1 < S < 15 in the James River estuary, 15 < S < 30 in the Southern Chesapeake Bay and S > 30 in the Atlantic Ocean at the Bay mouth, respectively. Total I behaved conservatively (i.e., no evidence of consumption or production) during estuarine mixing during both cruises. In the southern Chesapeake Bay, total inorganic I was also approximately conservative. The primary process affecting the speciation of dissolved I was the conversion of IO₃⁻ to I⁻. In the James River estuary, there were indications of the conversion of both IO₃⁻ and I⁻ to DOI. The concentrations of total I, IO₃⁻, I⁻ and DOI in James River water were 0.121, undetectable, 0.068 and 0.053 µM, respectively. These concentrations of total I and I⁻ are significantly higher while that of IO₃⁻ is noticeably lower than those used presently for estimating global riverine input of these I species to the oceans. The riverine flux of DOI to the oceans is presently unknown.